1. Field of the Invention
This invention relates generally to an apparatus for the chemical vapor deposition of one or more coating constituents on a substrate e.g. glass, and more particularly to an apparatus for producing a coating with a nonuniform composition.
2. Description of the Relevant Art
It is known in the art that when a film of a transparent metal oxide, such as tin oxide, is deposited on a glass substrate, the coated glass substrate has non-uniform light-reflection across the visible spectrum because of the difference in the refractive index between the metal oxide and the glass substrate. In addition, when the thickness of the metal oxide coating is not uniform, the coating tends to display a multiplicity of interference color effects commonly referred to as iridescence. Such iridescence effects render the coated glass aesthetically unacceptable for most architectural applications. Thus, various methods to mask such iridescence effects and/or reduce reflectance have been proposed.
One technique for minimizing or eliminating the difference of the refractive index between a metal oxide and a glass substrate is disclosed in U.S. Pat. No. 3,378,396 to Zaromb wherein a glass substrate is coated by simultaneously directing separate sprays of a tin chloride solution and of a silicon chloride solution onto a stationary heated glass piece in an oxidizing atmosphere e.g. air. The heat of the glass piece thermally converts the metal chlorides to their metal oxides. The ratio of the sprays to each other are gradually varied to vary the ratio of the weight percent of the metal oxides in the coating. The resultant coating has a continuously-changing composition throughout its thickness, e.g. near the glass-coating interface, the coating is predominantly silicon oxide, the surface of the coating furthest from the glass-coating interface is predominantly tin oxide and there-between the coating is made up of varying weight percent amounts of silicon oxide and tin oxide. Strong in his publication entitled "Practical Applications of High and Low-Reflecting Films on Glass", pages 441-443 of Le Journal de Physique et Le Radium, Vol. 11, July 1950, teaches that a coating technique similar to that taught by Zaromb reduces the iridescence of the coated article.
Additional techniques using the Zaromb teachings to coat a moving substrate are taught in U.S. Pat. Nos. 4,206,252 and 4,440,882. These patents further teach the depositing of a second coating composed of fluorine-doped tin oxide on the first coating of the type taught by Zaromb.
Gordon, in U.S. Pat. Nos. 4,187,336 and 4,308,316 discloses the reduction of iridescence of a tin oxide coating on a glass substrate by the use of an intermediate coating between the tin oxide coating and the glass substrate having a thickness and refractive index satisfying the optical equation: the refractive index of the intermediate coating is equal to the square root of the refractive index of the glass substrate times the refractive index of the tin oxide coating.
U.S. Pat. Nos. 4,377,613 and 4,419,386 to Gordon disclose a reduction in iridescence arising from a tin oxide film on a glass substrate by providing two intermediate coating layers between the glass substrate and the tin oxide. The intermediate layer next to the surface of the glass substrate has a high refractive index, while the intermediate layer farther from the surface of the glass substrate and next to the tin oxide film has a lower refractive index.
In general, the patents discussed above, except for U.S. Pat. Nos. 4,206,252 and 4,440,822, teach coating a stationary glass substrate. Apparatuses for coating a moving glass substrate with metal oxides are disclosed in the above discussed U.S. Pat. Nos. 4,206,252 and 4,440,882 to Gordon, and in U.S. Pat. No. 4,853,257 to Henery and U.S. Pat. No. 4,386,117 to Gordon.
In U.S. Pat. Nos. 4,206,252 and 4,440,882, the underside of a moving hot glass ribbon is coated by directing coating compositions containing metal compounds toward the ribbon surface, whose compounds are converted to their corresponding metal oxides.
U.S. Pat. No. 4,853,257 discloses an apparatus for depositing a low emissivity film on a glass ribbon by directing metal-containing coating reactants in vapor form onto the upper surface of a glass ribbon while the glass ribbon is supported on a molten metal bath contained in a non-oxidizing atmosphere. The carrier gas, the unreacted coating composition and any decomposition by-products are removed from the coating zone by an exhaust orifice on each side of, and equidistant from, the position where the coating reactants in vapor form are directed toward the glass ribbon.
U.S. Pat. No. 4,386,117 discloses a process for depositing a mixed metal oxide coating on a glass substrate by directing a gaseous mixture onto a moving glass ribbon and then exhausting gases from the coating zone at two locations equidistant from the entry of the gaseous mixture into the coating zone.
Although each of the apparatuses and processes taught in the above-discussed patents is acceptable for its intended purpose, there are limitations when the apparatuses and processes are used to apply the coating of Zaromb to a moving heated glass substrate, e.g. a glass ribbon supported on a molten metal bath contained in a non-oxidizing atmosphere. It would be advantageous, therefore, to provide apparatuses and processes to deposit the coating of Zaromb on a moving heated substrate as well as the metal containing precursors used in the preparation of the coating.
One of the limitations of the presently available vapor coating system for coating a glass ribbon moving at fast speeds e.g. about 600 inches/min. (15.24 meters/min.) is that the vapor coating mixture does not have sufficient time to deposit a coating of acceptable thickness on the glass ribbon. The article entitled "The LPCVD of Silicon Oxide Films Below 700.degree. F. (400.degree. C.) From Liquid Sources" by A. K. Hochberg and D. L. O'Meara published in J. Electrochem. Soc. Vol. 136, No. 6, June 1989 copyrighted by The Electrochemical Society, Inc. pps. 1843 and 1844 teaches the use of trimethylphosphite to accelerate coating deposition below 750.degree. F. (400.degree. C.). The publication "User's Guide For: Glass Deposition with LTO-410.TM. Source Material" by Dr. A. Hochberg and Dr. B. Gelernt, copyrighted 1990 by Schumacher of Carlsbad, Calif., 92009 teaches that the LTO-410 process is not significantly changed with the addition of trimethylphosphite.
Although the use of accelerants is taught, there are no teachings that such accelerants are beneficial at elevated temperatures e.g. above 750.degree. F. (400.degree. C.). Therefore it would be advantageous to provide accelerants for coating systems that operate at temperatures above about 1000.degree. F. (536.degree. C.).